As shown in FIG. 1, an adjustable wrench of the prior art has a fixed jaw 1 and a movable jaw 2. A mouth 3 is formed between the fixed jaw 1 and the movable jaw 2 such that the size of the mouth 3 is adjusted by the movable jaw 2.
Such a prior art adjustable wrench as described above is defective in design in that the movable jaw 2 is prone to move aside at the time when a nut or the head of a bolt is held and turned in the mouth 3. The mouth 3 has an open end. Certain adjustable wrenches of the prior art have a closed mouth, a handle, and a jaw head. The jaw head is provided longitudinally with a receiving hole. The jaw head is further provided axially with a round hole for pivoting an adjustment bolt capable of axial and linear motion. The planar portion of the front end of the adjustment bolt is used as a movable clamping surface for holding a nut or bolt in conjunction with the left side and the right side of the jaw head. The planar portion of the front end of the adjustment bolt is vulnerable to moving aside when it is rotated to make an axial displacement. As a result, the nut or bolt can not be securely held by the movable clamping surface.